My mesh's automatically deformed in the viewport after aplying a new material to some faces of it
while I can't do much without access to the .blend file, from viewing the screenshot it does seem that the strange interior geometry isn't part of the cup object (if it were, it would have edge lines on it since you are in edit mode). I would try leaving edit mode and trying to select it to see if it is a different model. If it is part of the cup, I would try recalculating the cup's normals by pressing "A" to select all faces, then "Alt + N", then selecting "Recalculate Outside" or "Set from Faces". 

I would also check the modifiers tab on the cup and see if you have any displacement or other modifiers which could be causing the effect.